The present invention in general relates to an image formation apparatus. More particularly, this invention relates to an image formation apparatus in which it is possible to effectively remove the unwanted toner from the intermediate transfer body.
An image formation apparatus configured as a multifunction machine is conventionally known. Such an image formation apparatus has functions of an electronic copying machine, a printer, and a facsimile, or at least two of the functions. In this type of image formation apparatus, toner images of different colors are primarily transferred superposedly onto its intermediate transfer body. The superposed toner images are then secondarily transferred collectively onto a recording medium, so that a color image can be formed.
Any residual toner existing on the intermediate transfer body, after the toner image is secondarily transferred onto the recording medium, is removed from the surface of the intermediate transfer body using a second cleaning device. This second cleaning device also removes any toner not having received the action by the secondary transfer unit on the intermediate transfer body. For example, when a recording medium is not successfully conveyed due to paper jam or the like during image formation, the operation of the image formation apparatus is stopped. The image forming operation is restarted after the unsuccessfully conveyed recording medium is removed, in other words, after the jammed paper is removed. When restarting the operation, the toner image, that is formed on the intermediate transfer body on the upstream side from a secondary transfer region in the direction of the movement of the intermediate transfer body, passes through the secondary transfer region without receiving the secondary transfer action, and the toner is removed from the surface of the intermediate transfer body by the second cleaning device. The second cleaning device removes not only the residual toner from the intermediate transfer body but also the toner not having received the secondary transfer action from the intermediate transfer body. However, if the efficiency with witch the toner is removed is less, the toner remaining on the intermediate transfer body is stuck on a next recording medium. This fact inevitably degrades the quality of the toner image on the recording medium, and makes background dirt more significant.
Therefore, in the conventional image formation apparatus, a voltage applied to the cleaning member is set as follows. The voltage is set in such a manner that any residual toner remaining on the intermediate transfer body and any toner, which has not received the secondary transfer action, on the intermediate transfer body can be most efficiently removed therefrom by the cleaning member of the second cleaning device. That is, the voltage is set in such a manner that the cleaning efficiency of the cleaning member becomes the highest.
However, there are problems with the above-mentioned method. If the voltage is set in the manner explained above, the small amount of toner remaining on the intermediate transfer body, due to unsatisfactory cleaning by the second cleaning device, is easily transferred onto the next recording medium. Resultantly, background dirt may occur on the next recording medium and the quality of the toner image secondarily transferred onto the recording medium may be degraded.
Another image formation apparatus has been proposed. In this image formation apparatus, a charger is provided on the downstream side from the secondary transfer region where secondary transfer of the toner image is performed in the movement direction of the surface of the intermediate transfer body. As a result, any residual toner on the intermediate transfer body is forcefully charged to a polarity opposite to its normal polarity. The residual toner is then electrostatically shifted to the surface of the image carrier in a primary transfer region where primary transfer of the toner image is executed. The shifted toner is then removed from the surface of the image carrier by a cleaning unit for cleaning the image carrier. This image formation apparatus stops removing the residual toner from the intermediate transfer body by the cleaning unit for the intermediate transfer body, returns all the residual toner to the surface of the image carrier, and removes the toner from the image carrier by the cleaning unit for the image carrier. Thus, the residual toner after secondary transfer deposited on the intermediate transfer body is a small amount. Therefore, it is possible to shift the toner to the image carrier and remove the toner therefrom efficiently by the cleaning unit for the image carrier.
However, according to the proposed image formation apparatus, it is difficult to remove the toner, which has not received the secondary transfer action by the secondary transfer unit, on the intermediate transfer body. That is, when the image forming operation is restarted after the jammed paper is removed, the amount of toner existing on the intermediate transfer body is much larger as compared to the residual toner remaining on the intermediate transfer body after secondary transfer. Further, this large amount of toner is strongly charged to the normal polarity because the toner has not received the secondary transfer action. In the image formation apparatus conventionally proposed, the large amount of toner charged to the normal polarity is also forcefully charged to the polarity opposite to the normal polarity by the charger. This charged toner is supposed to be electrostatically returned to the surface of the image carrier. However, it is difficult to charge the entire toner in the large amount, which has been strongly charged to the normal polarity, to the polarity opposite to the normal polarity by the charger. Accordingly, the large amount of toner that has not been shifted to the surface of the image carrier remains on the intermediate transfer body. This remaining toner is shifted onto a next recording medium, so that the recording medium may be soiled by the toner.
It is an object of this invention to provide an image formation apparatus that can more effectively reduce the amount of toner shifted to a recording medium out of the toner that remains on an intermediate transfer body without being cleaned as compared to the conventional case.
In the image formation apparatus according to this invention, the absolute value of a voltage to be applied to the cleaning member is set to a value greater than the absolute value of a voltage applied to the cleaning member at which toner removal efficiency, when the second cleaning device removes any toner on the intermediate transfer body not having received the secondary transfer action by the secondary transfer unit therefrom, is maximum.
At that time, it is advantageous to set the absolute value of the voltage to be applied to the cleaning member to a value 1.5 or more times the absolute value of the voltage applied to the cleaning member at which the toner removal efficiency, when the second cleaning device removes any toner on the intermediate transfer body not having received the secondary transfer action by the secondary transfer unit therefrom, is maximum.
Further, it is also possible to set the voltage to be applied to the cleaning unit so that the followings are obtained. The absolute value of an average charge amount of toner on the intermediate transfer body, after passing through the second cleaning device without receiving the secondary transfer action by the secondary transfer unit and before reaching the primary transfer region where the primary transfer is performed, becomes ⅕ to 4 times the absolute value of an average charge amount of the toner before reaching the second cleaning device. In addition, the charge polarity of the toner before reaching the second cleaning device is opposite to that of the toner after passing through the second cleaning device.
Further, it is advantageous that the cleaning member is formed with a cleaning roller which is rotatably driven, and the cleaning unit is formed with a blade which is in contact with the surface of the cleaning roller with pressure to scrape off the toner deposited on the surface of the cleaning roller. It is also advantageous that the surface roughness of the cleaning roller is set to a value equivalent to or less than the average diameter of toner particles.
Further, it is advantageous that the cleaning member is formed with a brush roller which is rotatably driven, and the cleaning unit is formed with a flicker which is in contact with a brush of the brush roller to flick off the toner stuck to the brush.
Further, it is advantageous that the cleaning member is formed with the brush roller which is rotatably driven, and the cleaning unit has a recovery roller which rotates while being in contact with the brush of the brush roller and electrostatically recovers the toner stuck to the brush and a blade which scrapes off the toner deposited on the surface of the recovery roller. It is also advantageous that the surface roughness of the recovery roller is set to a value equivalent to or less than the average diameter of toner particles.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.